BGP (Border Gateway Protocol) confederations are a method used to manage the complexity and scalability of BGP in large networks, particularly in networks with a large number of BGP sessions. A BGP confederation divides a single autonomous system (AS) into multiple smaller, manageable ASes. This approach helps in reducing the internal BGP (iBGP) full mesh requirement and simplifying the overall management of BGP.
Purpose of BGP Confederations #
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Manageability: Splitting a large AS into smaller ASes within a confederation makes the network easier to manage.
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Scalability: By reducing the number of iBGP connections required in a large network, confederations help in scaling BGP deployment more effectively.
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Routing Policy Control: Confederations provide more flexibility and control over routing policies and how routes are advertised within and outside the confederation.
How BGP Confederations Work #
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Sub-ASes: In a BGP confederation, an autonomous system is divided into multiple sub-ASes. Each sub-AS is a fully functional BGP AS, and it runs its own instance of BGP.
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Internal and External BGP: Within a confederation, BGP speakers use iBGP to communicate with BGP speakers in the same sub-AS and eBGP to communicate with BGP speakers in different sub-ASes of the same confederation. To the outside world, the confederation still appears as a single AS.
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Route Advertisement: Routes advertised between sub-ASes in a confederation are treated as eBGP routes, but they retain some iBGP characteristics, such as next-hop preservation.
Confederation Attributes #
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Confederation Identifier: The entire confederation is identified by a single AS number, which is seen by external BGP peers.
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Confederation Peers: Routers within a confederation treat other sub-ASes as external peers, but with special handling appropriate for a confederation.
Advantages of Confederations #
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Reduced iBGP Complexity: Reduces the number of iBGP sessions within each sub-AS, mitigating the full mesh requirement of iBGP.
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Flexible Policy Control: Allows more granular control over routing policies within an AS.
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Better Network Stability: Helps in isolating routing issues within a sub-AS, preventing wide-scale impact.
Confederations vs. Route Reflectors #
Confederations and route reflectors are both strategies to scale BGP in large networks. While confederations divide an AS into smaller ASes, route reflectors reduce the number of iBGP connections by reflecting routes within an AS. The choice between using confederations or route reflectors (or a combination of both) depends on the specific requirements and architecture of the network.
In summary, BGP confederations are a powerful tool for managing large BGP deployments, providing a way to scale and control routing in large networks more effectively. They offer a compromise between the scalability of eBGP and the simplicity of iBGP, making large-scale BGP networks more manageable and stable.